1. Field of the Invention
The present invention relates generally to communication systems employing amplification devices. More particularly, the invention pertains to a non-linear predistortion or postdistortion generator for coupling in-line with an optical receiver, optical laser transmitter or an amplifier to minimize second and third order distortion caused by the signal processing.
2. Description of the Related Art
Analog intensity modulation of a distribution feedback (DFB) laser is a widely used technique to transmit analog signals, such as sound or video signals and data, on optical fibers over a long distance. Optical detector also is widely used in fiber optic link. The performance of DFB lasers and optical detectors are limited by their distortion performance. Improving second order and third order distortion performance can greatly improve the entire system performance and increase the entire system dynamic range.
Amplifiers are also widely used in many types of communication applications. Although it is preferable to keep amplifiers within their linear range of operation, it has been increasingly necessary to extend the operation of amplifiers into high power and high frequency regions of operation. Typically, the output power of an amplifier is limited by the non-linearity of the active devices, including bipolar transistors and FETs. These non-linearities result in distortions which are impressed upon the signal being amplified. Reducing the non-linear distortions of an amplifier results in increases of the output power, the system dynamic range and the carrier-to-noise ratio. Accordingly, minimizing distortions and achieving linear frequency response is paramount to efficient amplifier operation.
Minimizing distortion is particularly important when a series of amplifiers is cascaded over a signal transmission path, such as a series of RF amplifiers in a CATV transmission system. Disposed throughout a CATV transmission system are RF amplifiers that periodically amplify the transmitted signals to counteract cable attenuation and attenuation caused by passive CATV components, such as signal splitters and equalizers. The RF amplifiers are also employed to maintain the desired carrier-to-noise ratio. Due to the number of RF amplifiers employed in a given CATV transmission system, each RF amplifier must provide minimum degradation to the transmitted signal.
Many amplifiers are subject to a wide range of ambient operating temperatures. These temperature changes may affect the operating characteristics of certain electronic components within the amplifier, thereby inducing additional distortions. A temperature range of xe2x88x9240xc2x0 C. to +85xc2x0 C. is not uncommon for many amplifier applications in a communication environment. To ensure consistent performance over the operating bandwidth, and to minimize resulting distortions, an amplifier must be designed for a broad range of ambient operating temperatures.
The distortions created by an amplifier which are of primary concern are second (even) and third (odd) order harmonic intermodulation and distortions. Prior art amplifier designs have attempted to ameliorate the effects of even order distortions, such as composite second order (CSO) distortion, by employing push-pull amplifier topologies, since the maximum second order cancellation occurs when equal amplitude and 180xc2x0 phase relationship is maintained over the entire bandwidth. This is achieved through equal gain in both push-pull halves by matching the operating characteristics of the active devices. In some cases, second order correction is still needed in order to get good CSO performance. Many prior art designs include the use of a separate second order distortion circuit to provide such the correction for CSO.
However, odd-order distortion is difficult to remedy. Odd-order distortion characteristics of an amplifier are manifest as cross modulation (X-mod) and composite triple beat (CTB) distortions on the signal being amplified. X-mod occurs when the modulated contents of one channel being transmitted interferes with and becomes part of an adjacent or non-adjacent channel. CTB results from the combination of three frequencies of carriers occurring in the proximity of each carrier since the carriers are typically equally spaced across the frequency bandwidth. Of the two noted distortions, CTB becomes more problematic when increasing the number of channels on a given CATV system. While X-mod distortion also increases in proportion to the number of channels, the possibility of CTB is more dramatic due to the increased number of available combinations from among the total number of transmitted channels. As the number of channels transmitted by a communication system increases, or the channels reside close together, the odd-order distortion becomes a limiting factor of amplifier performance.
There are three basic ways of correcting distortion created by a non-linear device (NLD): 1) reduce the signal power level; 2) use a feed forward technique; and 3) use a predistortion or postdistortion technique. The first method reduces the signal power level such that the NLD is operating in its linear region. However, in the case of an RF amplifier this results in very high power consumption for low RFoutput power.
The second method is the feed forward technique. Using this technique, the input signal of the main amplification circuit is sampled and compared to the output signal to determine the difference between the signals. From this difference, the distortion component is extracted. This distortion component is then amplified by an auxiliary amplification circuit and combined with the output of the main amplification circuit such that the two distortion components cancel each other. Although this improves the distortion characteristics of the amplifier, the power consumed by the auxiliary amplification circuit is comparable to that consumed by the main amplification circuit. This circuitry is also complex and very temperature sensitive.
The third method is the predistortion or postdistortion technique. Depending upon whether the compensating distortion signal is generated before the non-linear device or after, the respective term predistortion or postdistortion is used. In this technique, a distortion signal equal in amplitude but opposite in phase to the distortion component generated by the amplifier circuit is estimated and generated. This is used to cancel the distortion at the input (for predistortion) or output (for postdistortion) of the amplifier, thereby improving the operating characteristics of the amplifier.
One such distortion design, as disclosed in U.S. Pat. No. 5,703,530 and shown in FIG. 1, relies upon a traditional xcfx80-attenuation network and a delay line for gain compensation; and a diode pair coupled with a delay line for distortion and phase compensation. This circuit generates a distortion that is equal in amplitude but opposite in phase to the distortion introduced by the amplifier. Plots of the distortions contributed by the distortion generator and the distortions manifest by the amplifier are shown in FIGS. 2 and 3. As shown, the distortion signal compensates for the distortions generated by the amplifier. However, the use of delay lines in such a manner is impractical since delay lines are physically large, are difficult to adjust and the results are inconsistent across a wide frequency range. Additionally, both amplitude and phase information are required for correct compensation. The ""530 patent also states that the system disclosed therein is not ideal for certain application, such as predistortion for CATV RF amplifiers, due to the excessive losses introduced by the distortion circuit.
An inline predistortion design, as disclosed in U.S. Pat. No. 5,798,854, provides compensation for NLDs by applying a predistorted signal equal in magnitude but opposite in phase to the distortion produced by the NLD. However, the circuitry disclosed therein is not matched to the NLD. Additionally, the ""854 patent presents a design that is typical of the prior art in the use of a high resistance bias for the diodes. This will reduce the correction efficiency and increase the effects of temperature upon the circuit.
Prior art designs also use separate correction circuits to correct for second and third order distortions if both types of corrections are required. This increases the cost of the overall circuit design and also generates more circuit losses.
Accordingly, there exists a need for a simple distortion generator which counteracts the distortion created by an NLD. The circuit should not introduce additional signal delay and should operate over a wide frequency bandwidth and wide ambient temperature range.
The present invention is an in-line predistortion or postdistortion generator for coupling in-line with an NLD to produce an output signal of useful amplitude, but with low composite second order, composite triple beat and cross modulation distortions. The distortion generator comprises an instant controlled non-linear attenuator which utilizes the non-linear current flowing through a pair of diodes to provide the proper amount of signal attenuation over the entire frequency bandwidth. The distortion generator circuitry is always matched to the NLD, thereby ensuring a frequency response that is predictable and predefined. The distortion generator permits selective adjustment of the non-linear current flowing through the diodes to create a second order distortion. The distortion generator also includes a temperature compensation circuit to ensure consistent operation throughout a wide temperature range.
Accordingly, it is an object of the present invention to provide a temperature compensated distortion generator which minimizes composite second order, cross modulation and composite triple beat distortions manifested by an NLD such as an RF amplifier, a laser diode or a photodetector.
Other objects and advantages of the of the present invention will become apparent to those skilled in the art after reading a detailed description of the preferred embodiment.